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RegDwigнt♦Dec 5 '12 at 9:54

3 Answers
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Most is what is called a determiner. A determiner is "a word, such as a number, article, personal pronoun, that determines (limits) the meaning of a noun phrase." Some determiners can only be used with either a countable noun or an uncountable noun, while others, like most, can be used with both countable and uncountable nouns. Uncountable nouns usually take a singular verb.

So, in your example, this is the correct form for your sentence: "Most companies are private." The plural noun is needed after the word most (company, referring to a business entity, is a countable noun, with a standard plural form [companies]), and then the plural verb is needed to construct a grammatically correct sentence.

But a sentence such as, "Most information is now obtained from the Internet." would take the singular verb (since information is considered an uncountable noun).

Interestingly, company can be an uncountable noun, but it then has a different definition than in your example. For example, "
Most company leaves after two or three days," is a perfectly acceptable sentence. In this case, company has the definition of a social gathering of guests or companions.

@EdwinAshworth, Can you give an example or three. I almost put that, but then couldn't think of an example that took a plural, off the top of my head.
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JLGDec 5 '12 at 17:46

From wiki.answers.com/Q/… : SOME UNCOUNTABLE NOUNS ARE PLURAL. These have no singular form. e.g. clothes, groceries, thanks, jeans, police, trousers, scissors. You may want to disagree with some of the list - I'm sure I've heard of 'a scissor' in US usage. And it's a syntactic feature - I'm sure the Chief Superintendent can readily find out how many police officers he's got under him, and 30 children will need 30 pairs of scissors. Furniture, clergy, people, headquarters, premises, means and cattle are some others.
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Edwin AshworthDec 5 '12 at 21:17